Although polymers comprising both esters and amides have been known for some time, they were thought to have very high melting points and not to exhibit the low temperature flow properties characteristic of known thermotropic polyesters which make them melt processable. For example, see U.S. Pat. No. 3,926,923 in which earlier work by Laakso et al. was discussed and the high melting point of their polymers was noted. The polymers of the patent have a repeating unit which contains three aromatic rings and one C.sub.2 -C.sub.8 alkylene group. There is no mention of the potential for exhibiting thermotropic polymorphic properties.
In U.S. Pat. No. 4,272,625 poly(esteramides) are disclosed which do exhibit thermotropic properties. This was accomplished by forming polymers having a repeating unit which includes residues of p-aminophenol and/or p-N-methylaminophenol and residues of carboxylic acids and which is characterized as having " . . . a balance of linear difunctional residues and dissymmetric difunctional residues . . . ". It is stated that "The linear difunctional residues contribute the anisotropic melt-forming property of the poly(esteramide) and the dissymmetric difunctional residues contribute the ability of the poly(esteramide) to melt below its decomposition temperature." In disclosing specific compounds found useful in making such polymers, the patentees clearly indicate that only aromatic residues are useful but that not all aromatic compounds can be used. Thus, one skilled in the art would not expect that non-aromatic residues could form thermotropic polymers based on the teaching of the U.S. Pat. No. 4,272,625.
Varshney in JMS-Rev. Macromol. Chem. Phys., C26(4), 551-659 (1986) makes reference to the patentees of U.S. Pat. No. 4,272,625 and states that "Incorporation of the amide group into the main chain, which increases interchain bonding, tends to increase crystalline phase stability and elevate melting points, thus preventing the observation of a liquid crystalline phase unless the symmetry of the system is reduced." An example of an unsymmetrical repeating unit is given which includes three aromatic rings and an alkylene group containing 2 to 12 carbon atoms. There is no indication that polymers containing symmetrical repeating units in which single aromatic rings are regularly alternated with alkylene groups would have thermotropic properties. The present invention discloses such polymers.
U.S. Pat. No. 4,182,842 discloses a poly(esteramide) having improved physical properties which are prepared from a polyester and a p-acyl aminobenzoic acid. Although the divalent radicals appear to be similar to those in the present invention, the actual repeating structure of the patentees' polymer is clearly different since it contains an aromatic radical designated A. It is not clear from the patentees' discussion whether or not their polymers are thermotropic in nature. Hot melt polymerization procedures such as employed by Laakso and in U.S. Pat. No. 4,182,842 tend to form random poly(esteramides) and not highly regular ones with strict alternation of the amide and ester pairs. The randomness is known to result from two effects: (1) initial placement of radical containing acid and amine groups is random and leads to randomness in the repeating units; (2) even if placed in regular fashion, the high temperature polymerization tends to randomize the system by transesterification and transamidation processes.